For single- or multicolor printing of a printing substrate (for example a single sheet or a band-shaped recording medium) made of the most varied materials (for example paper or thin plastic or metal films) it is known to generate image-dependent potential images (charge images) on a charge image carrier (for example a photoconductor) that correspond to the images to be printed, comprising regions to be inked and regions that are not to be inked. The regions of the potential images that are to be inked are made visible via toner with a developer station (inking station). The toner image is subsequently printed on the printing substrate.
A liquid developer containing toner and carrier fluid can thereby be used to ink the potential images.
A method for electrophoretic liquid developing (electrographic developing) in digital printing systems is known from, for example, WO 2005/013013 A2 (US 2006/0150836 A1). A carrier fluid containing silicon oil with ink particles (toner particles) dispersed therein is thereby used as a liquid developer. The supply of the liquid developer to the charge image carrier can occur via an applicator roller to which the liquid developer is supplied by a raster roller at which a chamber blade is arranged. The toner images are transferred via a transfer unit from the charge image carrier to the printing substrate. The transfer unit has a transfer element (for example a transfer roller) that transports the toner images to the transfer printing point in a transfer printing station and there interacts with, for example, a corona device in order to transfer the toner images onto the printing substrate. The transfer unit can additionally have a cleaning unit to clean the transfer element, which cleaning unit is realized as a blade, roller or formed fabric or fleece cleaner.
In printers that operate with liquid developer, the cleaning of the transfer element poses increased demands. Since only a little carrier fluid is provided at the transfer printing to the printing substrate for the toner transport to the printing substrate, the danger exists that toner fastens to the transfer element under pressure at the transfer printing point. Due to the small amount of carrier fluid, the danger moreover exists that a relatively large amount of toner remains on the transfer element. This toner (called residual toner of a residual image in the following) must be cleaned off the transfer element before the next transfer of a new print image. Otherwise a shadow image of the previously printed print image is received together with the new print image.